Well do you plan to do other things with your server box like adding a plex server?
If not then I can’t suggest using FreeNAS as your first linux os. I would try out either Ubuntu or Lubuntu desktop versions to learn how to use linux.
What kind of machine are you using? Is it a prebuild system, laptop or did you build your own system?
Also if possible, do watch this video for an introduction to linux. It is an hour long and it goes in-depth of what linux is and how to use it. https://youtu.be/YdtfuAJUce4?t=182
Also give this a read too: Basic skills and interests you'll need to be a MineOS admin